The present invention relates to a method for refining inorganic short fibers such as carbon nanotubes (CNTs).
The CNTs generated by an arc discharge method and a laser evaporation method generally include impurities, which are not CNT. For example, such CNTs include carbonaceous matter, such as graphites and fullerenes. In a case where metal catalysts are used to generate the CNTs, the CNTs further include the metal catalysts as impurities.
Japanese Laid-Open Patent Publication No. 8-198611 discloses a method for removing impurities included in CNTs. In the method of the publication, a mixture of CNTs and impurities goes through a centrifugal separation or a floatation to remove carbonaceous matter other than the CNTs. If CNTs include metal catalysts, acid is added in the mixture of the CNTs and the impurities to dissolve and remove the metal catalysts. Alternatively, the mixture of CNTs and impurities is passed through a magnetic field. However, in the above mentioned methods, CNTs and impurities are not reliably separated. Therefore, CNTs are not refined with a high yield rate.